1. Field of the Invention
The present invention concerns a microwave array antenna, for example a linear array adapted to be disposed along the focal line of a cylindrical-parabolic reflector.
2. Description of the Prior Art
Array antennas are designed to produce adaptive diagrams from a plurality of unit sources such as horns, helixes, dipoles, patches (small conductive patterns of rectangular shape, for example, etched on a substrate), etc.
By combining each unit source with a variable phase-shifter an electronically scanned antenna whose beam can be "depointed" (in other words: scanned) very quickly is obtained.
The simplest array antenna is the conventional linear array antenna which comprises in a single line a smaller or greater number of identical unit sources spaced at a regular pitch, the pitch being the distance from the centre of one source to that of the adjacent source.
By producing an array in a similar manner but in two orthongonal dimensions rather than a single dimension a "plane array" is obtained, often rectangular in shape, possibly with cut-off corners.
Similarly, by adopting a hexagonal grid it is possible to produce an array in the form of a plane body of revolution.
The drawback of all these regular pitch array antennas is that a large antenna requires a very large number of unit sources, to the point that the cost of an antenna of this kind can become prohibitive.
To reduce the number of unit sources some authors have considered the creation of "thinned" or "gapped" arrays by eliminating some sources either randomly or according to a deterministic law established mathematically on the basis of the theory of antennas, the number of sources removed increasing towards the edges of the array antenna. In all these implementations the unit sources of the array are identical to each other.
Such "thinning" reduces the number of unit sources without deteriorating the shape of the main lobe or causing "array lobes" to appear in the radiation pattern of the antenna (in other words, peaks in unwanted directions). Unfortunately this significantly reduces the gain of the antenna, which falls by 10 log R where R is the proportion of sources remaining: if half the unit sources are removed, the total antenna gain is reduced by 3 dB.
In many applications this degree of gain loss is prohibitive:
for a telecommunication transmit antenna, to maintain the same link balance it would be necessary to double the transmitted power, which is rarely possible; PA1 for a radar antenna, the gain of which is relevant both to transmission and reception, it would be necessary to quadruple the transmitted power.
The invention is directed to remedying these drawbacks.